1. Field of the Invention
This invention relates to a method and apparatus for processing digital video signals, e.g. digital colour TV signals, and in particular to keying of such signals.
2. Description of the Related Art
A particular problem with keying techniques (the overlaying of different scenes) in digital video signals is the observed "stairstepping" of pixels at the edges of the key--for example at the edges between two displayed scenes on a single TV screen or at the edges of graphics as displayed on a computer monitor. This stairstepping is particularly apparent in colour video, where there is intended to be an abrupt change of colour at the edges of the key.
This problem is less acute with analog video signals, as the key signal is a smooth waveform transition across the key edge. It is possible to clip or limit the signal at any point over this transition so as to get a sharp edge to the keyed pictures. With digital key signals this is not possible. The frequency of digitally sampling the key is not fast enough to provide an accurate representation of the transition waveform, with the result that the transition at the edge between two scenes is made on a pixel-to-pixel basis whereas a more accurate key would require faster sampling (i.e. smaller pixels)--which is currently not possible. This key switching on a pixel-by-pixel basis causes the "stairstepping" phenomenon described above.